The beneficial qualities of Vitamin D are well documented and accepted. For example, Vitamin D regulates the calcium level in the body and is responsible for depositing calcium and phosphorous into bone and the blood. Too little Vitamin D can cause soft bones, muscle weakness, poor growth, bone fractures, and secondary hyperthyroidism. Although vitamin supplements in tablet form may be taken, Vitamin D is easily obtainable from the diet, especially a diet containing Vitamin D enriched foods.
Vitamin D is a fat soluble vitamin produced from ergosterol and 7-dehydrocholesterol following exposure to sunlight. It is essential for the prevention of rickets, a disease in which the matrix of new bone is not mineralized. The most common biologically active forms of Vitamin D are previtamin D2 and D3, and vitamin D2 and D3. Previtamin D2 and vitamin D2 are produced from ergosterol and are biologically active in humans, cattle, swine and other mammals, but not in poultry. Previtamin D3 and vitamin D3 are biologically active and are produced in the skin of many animals following irradiation of 7-dehydrocholesterol. The other isomeric forms of vitamin D show no significant biological activity.
The structure of vitamin D2 shows a double bond at the C22 position and a methyl group at the C24 position that vitamin D3 lacks. For both types of vitamin D, the conversion of the previtamins to the active form involves a rupture of the ring and loss of the true sterol identity. As a result of the ring rupture 64 different isomers are theoretically possible. Only a few of the isomers occur naturally or during the synthesis of vitamin D.
The isomerization of vitamin D to previtamin D occurs in solution. The proportions depend on the temperature of the system and the time allowed for equilibrium. When vitamin D is prepared by irradiation the products include other isomeric forms such as tachysterol and lumisterol. These forms are not biologically active. Light, high temperatures and iodine also catalyze the conversion of the biologically active forms of vitamin D to inactive forms.
One excellent source of Vitamin D is liquid nutritional products which are fortified with Vitamin D. The term liquid nutritional product is meant to include any liquid consumed as part of a diet, which liquid has significant nutritional qualities. Examples include infant formula, milk, and even high caloric liquids such as Exceed.RTM. and Ensure.RTM., which are manufactured by the Ross Laboratories Division of Abbott Laboratories, Columbus, Ohio U.S.A.
With respect to infant formula, Vitamin D levels in infant liquid nutritional products are federally regulated in the U.S.A. by the Infant Formula Act (IFA). Levels above or below those set forth in the IFA may not legally be offered for sale. There are basically three types of infant formula: milk-based, soy-based, and protein hydrolysate-based. Some infants' immune systems experience an undesirable reaction to the protein found in milk-based or soy-based formula. The form of reaction may include one or more of the following: rash, dermatitis, diarrhea, crying or vomiting. In a protein hydrolysate-based liquid nutritional product for infants the nitrogen source is hydrolyzed protein or a free amino acid. Hydrolyzed protein is protein which has been broken down into much smaller peptides. Infants who experience adverse reactions to protein often are able to digest hydrolysate-based liquid nutritional products. As used herein "protein" is understood to relate to sources of proteins from plants and from animals.
However, protein hydrolysates have a major problem when it comes to Vitamin D. Vitamin D undergoes significant degradation. Until now the precise reason has not been generally known. However, it has been discovered that the reasons include: (1) the hydrolyzed protein source, (2) the method(s) of hydrolysis and (3) the presence of cysteine and its dimer, cystine (free or combined). Typical sources of hydrolyzed protein include casein, soy, whey and rice proteins. In fact, research has shown that the more hydrolyzed the protein, the worse the degradation. Furthermore, with respect to the presence of cysteine and cystine, it was discovered that cystine, and cysteine (which oxidizes to cystine, its dimer) induce Vitamin D degradation, typically by causing isomerization.
In view of the problems with Vitamin D degradation in liquid nutritional products having hydrolyzed protein or free amino acids as a nitrogen source, and by this is meant that hydrolyzed protein or free amino acids comprise the principal nitrogen source of the liquid nutritional product, some products in the liquid nutritional product industry have been over fortified with Vitamin D, apparently in an attempt to meet the label claims for Vitamin D content in the long term. However, as has been mentioned above, acceptable upper limits of Vitamin D concentration exist, since too much Vitamin D can cause hypercalcemia, hypercalciuria, urinary tract stones, extraskeletal calcifications, and malfunction of the kidneys and other organs.
If the Vitamin D concentration in a packaged liquid nutritional product exceeds the limits, the product may not be offered for sale until the Vitamin D concentration, through degradation, drops to an acceptable level. Similarly, if the Vitamin D concentration drops below acceptable lower limits, the product must be removed from sale to the public. The costs associated with replacing, shipping, and overall monitoring of the Vitamin D level in liquid nutritional products subject to significant Vitamin D degradation is enormous.
It is thus apparent that the need exists for a method to improve the stability of Vitamin D in the presence of protein hydrolysates, especially liquid nutritional products. Additionally, it should be understood the references to protein hydrolysates refer to partially or totally hydrolyzed proteins.